In the past years, vibratory sources or seismic vibrators, commonly identified by the trademark Vibroseis, became a dominant energy source for seismic surveys, particularly for land prospecting.
The principles of vibratory seismic are well known and compiled for example in: Geophysics reprint series No. 11 ("Vibroseis"), edited by R. L. Geyer, Society of Exploration Geophysicists, Tulsa, 1989, pp. 4-53. Typically a number of vibrators emit a long swept frequency signal in the seismic frequency range. The emitted signal (after reflection and refraction within the earth) is received by detectors located along a spread. To yield the response signal of the earth, the received signal is cross-correlated with the emitted swept frequency signal or a proportional signal (pilot sweep).The seismic record thus yielded is then processed into a representation of a cross-section of the earth using well-known computing techniques.
Practical applications of vibratory sources however suffer from various limitations caused for example by imperfect sweeping control and generation of higher harmonics of the sweep signal.
Several techniques to overcome those limitations have been applied. For example, it is known to use multiple vibratory sources in order to increase the energy transmitted to the earth. In a basic variant all sources are positioned at substantially the same position and are controlled to emit simultaneously the same signal.
Further variants of simultaneous vibroseis techniques involve operating a number of vibrators at different positions simultaneously, with the sweeps being designed so that the contribution from the individual vibrators may be separated through processing. In the simplest variant, two seismic record are acquired from two vibrators: the first records is gathered with both sources vibrating in phase, the second with both sources out of phase. The contribution of the first vibrator is derived by summing the two records, the contribution of the second by taking their difference. A more evolved method of separating multiple vibratory sources is described in the U.S. Pat. No. 4,175,020. Therein, n sources are positioned at n different positions, and all of the sources simultaneously transmit energy to the earth. These simultaneous sweeps are repeated n times to complete an acquisition cycle. The initial or offset phase of the sweeps is varied among n selected phase angles during the n sweeps of a source to enable the signal emanating from each vibratory source to be separated from the signals emanating from the other sources. A seismic survey in accordance with this method the n sources are advanced in a parallel alignment.
Another similar technique, requiring four sweeps for each of the sources, is described in the U.S. Pat. No. 4,823,326. The acquisition technique also described therein includes the step advancing the vibratory sources in parallel along straight lines.
Most of the simultaneous vibroseis techniques have been developed for land acquisition. In marine seismics, all of the techniques which require the source to be stationary for more than one sweep are not applicable, as the boat which tows the source advances at constant speed. Examples of marine seismic acquisition techniques are described in the aforementioned volume No. 11 of the Geophysics reprint series on pages 593-618. Therein, four marine vibrators are suspended in pairs on cables over both sides of the recording boat with two of those vibrators located near the ships center, and the other two near the stern. All vibrators receive identical in-phase control sweeps. Usually a sweep of 7 s is repeated from 4 to 6 times per minute.
Furthermore, it is known from the U.S. Pat. No. 4,405,999, to survey an area by towing arrays of seismic sources over that area and firing the seismic sources successively at the same location. The known method is adapted to the use of impulse sources (airguns). It is important to note and commonly accepted in the seismic industry that the use of vibratory sources and impulse sources are unrelated technical fields, each requiring its specific signal acquisition and processing methods.
With most of the methods developed for land seismic acquisition not applicable, it is the object of the present invention to provide methods and apparatus for the simultaneous operation of vibratory sources which can be utilized in marine seismic.